Magnetic field compensation according to the feedback control principle is known. A spatial volume which is also referred to herein as a compensation volume is made virtually or substantially field-free, through destructive interference. To this end, a feedback sensor determines the magnetic interference field, for example the geomagnetic field, near the object to be protected and supplies this signal to a control unit. Based on the sensor signals, the control unit calculates a compensation current which is fed to the compensation coils. These coils will then generate a field which in the ideal case destructively superimposes to the interference field such that the amplitude of interference is minimized or at least substantially reduced.
Especially apparatus working with accelerated electrons, such as scanning and/or transmission electron microscopes, suffer from electromagnetic interference, because the latter directly affects the trajectory of the electrons required for imaging and thus the quality of the results.
Typical installations for such devices include a compensation structure in form of a three-axes twin-coil system, which is usually designed as a cube. In this case, the respective opposite sides of the cube are used as a pair of coils for generating a compensation field as homogeneous as possible.
Inside this volume enclosed by the coils the device to be protected is then installed so that it can be operated in a virtually field-free space.
According to another prior art, only a single coil is provided per spatial direction or per axis instead of a respective pair of coils per spatial direction. The compensation field is then much more inhomogeneous, however, it is usually sufficient and economizes three coil structures.
All prior art configurations provide a sensor within the volume, because the control generally operates according to a feedback process. Thus, besides the cables that may be laid on the floor and via which the compensation coils are provided or wound, there is a sensor in the immediate vicinity of the device. So due to the construction of the arrangement, the operating comfort of the device is impaired.